Compounds of the type PdX2(PR2(CH2)nPR2) are preferred catalysts for many types of carbonylation reactions of aryl and vinyl halides and sulfonates. As with many other organometallic compounds the standard method of preparation for these compounds is to prepare, isolate and purify the phosphine ligand and react it with a suitable metal precursor. A major part of such preparations is the isolation and purification of the phosphine ligand. The ligand itself may be highly reactive and likely to suffer from oxidation, yielding the unreactive phosphine oxide as an impurity, so additional precautions such as the formation of an acid salt (e.g. using HBF4) are needed to stabilise the ligand for storage and ease of handling.
There are few examples of preparative procedures for ligands PR2(CH2)PR2 in the literature. One example relates to the preparation of iPr2P(CH2)3PiPr2 (K. Tani et al, J. Organometallic Chem. 1985, 279, 87-101). The method recommends the reflux of a reaction mixture comprising ether and n-hexane as solvents for the completion of the reaction before the removal of solvents and distillation to isolate the product. The present inventors have found, however, that thermal decomposition of the product during distillation reduces the overall yield.
Alternative routes to these compounds exist, such as the photochemical hydrophosphination of primary phosphines H2P(CH2)3PH2 reported by Maier (Helvetica Chimica Acta, 1966, 49, 842) and Lindner et al (J. Organometallic Chem., 2000, 602, 173). This method is not applicable to all diphosphines and the latter group also report the preparation via Cl2P(CH2)3PCl2 by reaction with Grignard reagents to yield alkyl-substituted diphosphines (J. Organometallic Chem., 2000, 602, 173). However, these methods involve complex, often multi-step preparation and purification procedures and yield highly air-sensitive products.
Typical routes for the formation of Pd complexes are described by Lindner et al (J. Organometallic Chem., 2000, 602, 173). Combination of the diphosphine ligand with a palladium precursor such as PdCl2(PhCN)2 yields PdCl2(R2P(CH2)nPR2). Reaction with Pd(OAc)2 can be used to prepare acetate complexes, although these compounds are reported to slowly decompose after the removal of solvents (Lindner et al., J. Organometallic Chem., 2000, 602, 173 and Z. Csakai et al., Inorg. Chim. Acta 1999, 286, 93).